This Youtube Video shows a smoke stream/trail above the skyline before the explosion https://www.youtube.com/watch?v=U6aM7WHm-NA&feature=youtu.be
This video from Twitter Shows a smoke stream/trail above the RV on the left side of the frame just before the explosion https://twitter.com/zachjorgen/status/1342577378384609280?s=20
Here is a link from a TN Resident commenting about the same thing. https://thedonald.win/p/11RNokQgd6/nashville-incident-media-is-full/
Here is the loudspeaker video for others that have not seen it. https://thedonald.win/p/11RNokS3bm/breaking-audio-warning-was-being/c/
Just speculation.
Could be many things , could have been diesel fuel and ammonium nitrate with the RV caught on fire as well.
In the 3rd link , the poster said it looked like a rocket trail , I meant to write "rocket" not missile , and that he had vets saying it looked like that.